The present invention relates to surface mounted electrical components having improved retentive properties, and methods for mounting and retaining electrical components on substrates.
Various methods of electrically connecting components such as connectors and integrated circuit devices to printed boards are well known in the art. Surface mount technology is increasingly being employed as cost-effective method. Examples of surface mount technology include pin grid arrays (PGA""s), ball grid arrays (BGA""s), column grid arrays (CGA""s), and land grid array (LGA""s). Generally with such methods, a solder paste is introduced between surfaces to be joined, the solder paste reflowed, and then cooled to form a mechanical and electrically conductive joint.
As the name implies, ball grid arrays (BGA""s) utilize a grid or array of solder bumps or balls arranged on one side of an electrical component to effectuate electrical and mechanical connection with a printed board. The ball attachment sites are typically referred to as lands or pads. Solder past is disposed on the lands (and/or on the balls), such as through screening or masking techniques, and the electrical component is then positioned on the board so that the balls and lands are registered. The board is then processed at an elevated temperature sufficient to cause the solder and at least a portion or all of the solder ball to flow and fuse with the underlying land/pad to form the electrical interconnection. See, for example, U.S. Pat. No. 6,325,644 assigned to the assignee of this patent, and patents within that patent, all of which is incorporated by reference herein.
Locator pins extending from the electrical component may be employed to facilitate the initial alignment of the electrical component. The locator pins are inserted into through holes located in the board at locations along the periphery of the array of lands/pads. Pin diameters may be slightly larger than the corresponding through hole diameters so that an interference fit is achieved. Alternatively, the pin diameters may be smaller than the through hole diameters. In this scenario, adhesive or solder paste may be applied in and around the through holes to help retain the electrical component on the substrate.
By selecting locator pins with a relatively smaller diameter and applying solder within the through holes, the BGA balls are allowed to float(that is, not rigidly restricted) during a reflow process resulting in self-alignment and ultimately a low stress solder joint. Component retention however may be compromised with this configuration when the board is manipulated before the reflow heat is substantially dissipated, or when the board is rotated so that additional electrical components can be mounted on other available surface areas with subsequent reflow steps.
Accordingly, there is a need for a method of mounting electrical components on a substrate that provides for a low stress solder joint while effectively retaining the mounted components during ensuing double-sided handling and/or processing during a second reflow step.
Methods for mounting electrical components on substrates are provided by the present invention. In accordance with a first preferred embodiment of the present invention, there has now been provided a method for mounting and retaining an electrical component on a substrate that is subjected to at least two thermal cycles. The method comprises the steps of: disposing solder paste of a first solder composition into a retentive through hole of the substrate; inserting a retentive pin extending from the electrical component and having a relatively smaller diameter into the retentive through hole; subjecting the substrate to a first thermal cycle such that material associated with at least one of the retentive through hole and retentive pin migrates into the solder paste to create a mixed solder composition that has a higher melting point than the first solder composition; and then subjecting the substrate to a second thermal cycle having a peak temperature less than the melting point of said mixed solder composition.
In accordance with another preferred embodiment of the present invention, there has now been provided a method for making an electronic assembly. The method comprises the steps of: providing a circuit board including a retentive through hole having a wall; providing an electrical connector having a retentive pin extending therefrom, wherein the retentive pin has a diameter smaller than the retentive through hole and is made from a material comprising a precious metal; disposing solder paste of a first solder composition into the retentive through hole; inserting the retentive pin into the retentive through hole; and reflowing the solder paste such that at least some of the precious metal originally associated with the retentive pin migrates into the solder paste to create a mixed solder composition.
In accordance with yet another preferred embodiment of the present invention, there has now been provided a method for mounting electrical components on opposing sides of a substrate. The method comprises the steps of: providing a substrate including first and second sides and first and second retentive through holes; providing a first electrical component including a first retentive pin extending therefrom, wherein the first retentive pin has a diameter smaller than that of the first retentive through hole, and has plating material thereon; disposing solder paste of a first solder composition into said first retentive through hole; juxtaposing the first electrical component on the first side of the substrate, and inserting the first retentive pin into the first retentive through hole; heating the substrate such that at least a portion of the plating material migrates from the first retentive pin into the solder paste, wherein a mixed solder composition is created which has a higher melting point than that of said first solder composition; disposing solder paste of the first solder composition into the second retentive through hole; providing a second electrical component including a second retentive pin extending therefrom, wherein the second retentive pin has a diameter smaller than that of the second retentive through hole; juxtaposing the second electrical component on the second side of the substrate, and inserting the second retentive pin into the second retentive through hole; and heating the substrate to a temperature sufficient to reflow the first solder composition and below the melting point of the mixed solder composition.
Electronic assemblies are also provided by the present invention. In accordance with a preferred embodiment of the present invention, there has now been provided an electronic assembly comprising a circuit board including a retentive through hole having a wall; an electrical component including a housing and retentive pin extending therefrom mounted on the circuit board, wherein the retentive pin is disposed within said retentive through hole and has a diameter less than that of the retentive through hole; and reflowed solder disposed in the retentive through hole and surrounding at least a portion of the retentive pin, wherein the reflowed solder includes material originally associated with at least one of the retentive pin and the retentive through hole wall, and wherein the reflowed solder has a melting point greater than a melting point of pure solder.
In accordance with another preferred embodiment of the present invention, there has now been provided an electronic assembly comprising a circuit board including a retentive through hole; an electrical component including a housing and retentive pin extending therefrom mounted on the circuit board, wherein the retentive pin is disposed within the retentive through hole and has a diameter less than that of the retentive through hole; and reflowed solder disposed in the retentive through hole and surrounding the retentive pin, wherein at least about 0.5% by weight of said reflowed solder is an additive comprising a precious metal.
In accordance with yet another preferred embodiment of the present invention, there has now been provided an electronic assembly comprising a circuit board including a retentive through hole; an electrical component including a housing and retentive pin extending therefrom mounted on the circuit board, wherein the retentive pin is disposed within the retentive through hole, has a diameter less than that of the retentive through hole, and has plating material thereon; and reflowed solder disposed in the retentive through hole and surrounding the retentive pin, wherein the reflowed solder comprises at least about 0.5% by weight of the plating material that has migrated from the retentive pin.
These and various other features of novelty, and their respective advantages, are pointed out with particularity in the claims annexed hereto and forming a part hereof. However, for a better understanding of aspects of the invention, reference should be made to the drawings which form a further part hereof, and to the accompanying descriptive matter, in which there is illustrated preferred embodiments.